Transducer circuit and method of operation



Aug. 25, 1970 vERDlER 3,525,812

TRANSDUCER CIRCUIT AND METHOD OF OPERATION Original Filed Oct. 21, 1965Fees may nvuroe N a8 I i 56 fiunse I w ging-7o "11%1'54556 2 L rx 55 FINVENTOR. I l6, 3 JAMES EVERDIER United States Patent 3,525,812TRANSDUCER CIRCUIT AND METHOD OF OPERATION James E. Verdier, 453 PardeePlace, Dayton, Ohio 45431 Continuation of application Ser. No. 500,388,Oct. 21, 1965. This application May 8, 1969, Ser. No. 823,190

Int. Cl. H041 3/00 11.5.. El. ll'W- -ll 7 Claims ABSTRACT OF THEDISCLOSURE The invention relates to a loud speaker circuit in which theresistance drop across the speaker coil is measured and is fed back intothe driving voltage for the speaker coil after having been inverted soas to be 180 out of phase with the resistance drop across the speakercoil.

The present invention relates to sound producing systerns and inparticular to improvements therein. More particularly still, the presentinvention pertains to the reduction of distortion in sound producingsystems, especially in sound systems containing an amplifier and aspeaker of the moving coil type. Some systems are known which employ afeedback proportional to the movement of the speaker coil. These systemshave been called Motional feedback systems, and others have been calledAmplifiers with negative output impedance equal to the blocked impedenceof the speaker.

This application is a continuation of application Ser. No. 500,388,filed Oct. 21, 1965, now abandoned.

In this application, the drive amplifier and transducer (speaker) willbe considered the sound producing sys tern. The transducer is defined asa device that is powered by a current flowing through a wire that isdisposed in a magnetic field. The wire of the transducer (voice coil) isattached to a diaphram (cone) that is mounted in a box called anenclosure. The drive amplifier is defined as the electronic circuitrythat produce voltage gain, power and other signal preparation requiredto drive the speaker.

in a conventional sound producing system, the drive amplifier isdesigned to deliver a voltage that is a constant times the input voltageover some useable frequency range. This constant voltage generator, asit may be called, is connected directly to the speaker. At lowfrequencies, say, below 1000 cycles per second (c.p.s.) for speakerswith good efiiciency, the speaker is essentially a resistive load.Therefore, at low frequencies the current in the voice coil isessentially constant with frequency. Movement of the cone is thencontrolled mainly by this current, in combination with the mass andspring combination of the cone, and acoustical factors such as boxresonance and standing waves within the box. The combination of thesefactors cause the level of the sound that is produced to change asfrequency-is changed. In addition, the spring constant of the centeringdevice of the cone causes some intermodulation distortion.

Most attempts at improving the performance of this system have beenaimed at controlling the natural mechanical response of the cone. Theseattempts have ineluded very large boxes, boxes with tuned ports, exponential horns, acoustic suspension of the cone, large amounts of acousticaldamping, large cone mass, and improve ment of the quality of the driveamplifier. The result of these techniques has been either large physicaldimensions or low efiiciency. These techniques have thus, furthermore,been only moderately successful in producing a high quality soundproducing system.

Another approach to the problem of improving the performance of theamplifier-speaker combination is the use of inverse feedback that isproportional to the move- 3,525,812 Patented Aug. 25, 1970 ment of thecone. With large amounts of feedback and proper compensation for theradiation characteristics of the cone, a sound producing system can bemade that has low harmonic distortion and having a sound output verynearly equal over a wide range of frequencies, in small enclosures andwithout special acoustical absorption, tuned ports, or cones with largemass and acoustical suspension. The effects of standing waves, boxresonances and the cone resonance can be substantially reduced. Inshort, a system with improved performance can be made at less cost.

It is the particular purpose of this invention to make possible the useof large amounts of inverse feedback, proportional to the movement ofthe cone, while maintaining good amplifier-speaker system stability andwhile using conventional amplifier components. It is further the purposeof the invention to accomplish this feedback, using a conventionalspeaker, without adding any special devices to the speaker.

In order to accomplish movement feedback, a voltage proportional solelyto the movement of the cone must be produced. One method ofaccomplishing this is to mount a second coil and magnet in front of thespeaker. The second coil is attached to the cone. Movement of the secondcoil in a second magnetic field generates the desired voltage. Thismethod, however, has several drawbacks. It adds considerable cost to thespeaker, lowers the speakers efiiciency because of the increased conemass, and the mechanical connection between the drive coil and secondcoil contributes high frequency phase shift that may limit the amount ofthe feedback to a low value.

A voltage corresponding to that generated by the method described in thepreceding paragraph is generated within a conventional speaker in theform of the back induced voltage that results from the movement of thecoil in its magnetic field. This voltage is a part of the total voltageacross the speaker and accounts for some of the electrical impedance ofthe speaker and is proportional to the velocity of the cone. A method ofextracting this back induced voltage, using a bridge that is balanced tothe DC resistance of the speaker, is disclosed in US. Pats. 2,887,532 toWerner and 2,167,011 to Tellegen.

This method of generating a feedback voltage also has certain drawbacks.In order to maintain reasonable power efficiency, the resistor oppositethe speaker resistance in the bridge must be much smaller than theresistance of the speaker. As a result, the speaker back EMF is dividedby the ratio of speaker resistance to the bridge resistor and again by afactor of 2. Since the back EMF is a very small portion of the totalspeaker voltage at low frequencies, very high amplifier gain is requiredto convert the detected back induced voltage to a usable signal. Thecoupling phase shifts occurring in the required additional amplifiersmay prevent the use of large amounts of feedback without additionalpower supplies and direct coupled amplifiers, and adding considerablecost to the amplifier.

With the foregoing in mind, the primary object of the present inventionis a method of and a circuit for extracting the back EMF in a speaker ortransducer coil and utilizing the extracted back EMF for improving theopera tion of the speaker or transducer.

The exact nature of the present invention will be more clearlyunderstood upon reference to the following detailed specification takenin connection with the accompanying drawings in which:

FIG. 1 is a somewhat schematic block diagram illustrating one circuitaccording to the present invention;

FIG. 2 is a view similar to FIG. 1 but shows another circuit; and

FIG. 3 shows still another circuit arrangement according to the presentinvention.

FIGS. 1 and 2, are block diagrams of the method of extracting andapplying the back induced voltage of a transducer. Polarity of signals,relative to return are indicated by plus and minus signs. Plus signalsare generally in phase with the drive voltage, while negative signalsare generally out of phase with the drive voltage.

In FIG. 1, a drive voltage source, 1, is connected to the system returnwire 10, and to the input of a frequency compensation network 2, whichhas a connection to return line 10 and the output of which is connectedto voltage summer 6, where it is added to the voltage from a frequencycompensation network 4. The input of network 4 is connected to theoutput of voltage summer 3. Two inputs of voltage summer 3, areconnected to the voltage across the transducer coil 12, and another tothe output of amplifier 5.

The input of amplifier 5 is connected to a resistor that has one endconnected to the return wire of the systern. The input of 5 is alsoconnected to the secondary winding of transformer 8, the other end ofthe winding being connected to one side of the transducer coil. Theother side of the transducer coil is connected to the return wire 10.Amplifier 5 has a connection to return wire 10. The output of voltagesummer 6 is connected to the input of amplifier 7, which also has aconnection to the return wire 10. Its output is connected to the primarywinding of transformer 8, the other end of which is con nected to thereturn wire 10.

11 represents the enclosure of the speaker, and 13 represents themovable cone of the speaker that is connected to the coil 12, which isdisposed in a magnetic field.

In FIG. 2, a drive voltage source 14 is connected between return wire 21and a frequency compensation network 15, that has a connection to returnwire 21. The output of 15 is connected to one of the inputs of voltagesummer 16. The other input of 16 is connected to the output of afrequency compensation network 20, that has a connection to return wire21. The output of voltage summer 16 is connected to the input ofamplifier 17, which also has a connection to return wire 21. The outputof 17 is connected to a resistor 18, the other end of which is connectedto one end of the transducer coil. The other end of the transducer coilis connected to return wire 21. Amplifier 19 has a connection to thejunction of resistor 18 and coil 24, while its input is connected to thejunction of resistor 18 and amplifier 17. Its output is connected to theinput of frequency compensation network 20.

22 represents the enclosure of the speaker and 23 represents the movablecone that is attached to voice coil 24, which is disposed in a magneticfield.

At any one frequency the total voltage across the speaker voice coil 12or 24 can be considered to be the vector sum of the I R drop, or the DCresistance of the speaker, and the back induced voltage resulting frommovement of the voice coil wire in the magnetic field. For purposes ofthis discussion the inductance portions of the voice coil not in themagnetic field has been neglected since it is, in fact, negligible anddoes not represent the main effect. It will be referred to later.

The back induced voltage can be extracted from the total speaker voltageby adding to the total speaker voltage, an auxiliary voltage equal tothe I XR drop and out of phase with the total voltage. Since the addedvoltage cancels the I R drop in the speaker, the remaining voltage isonly the back induced voltage. A voltage equal to the I R drop isproduced by amplifying the voltage across a small resistor placed inseries with the speaker. Phase inversion is achieved within theamplifier or by placement of the resistor in the circuit.

FIG. 1 shows a block diagram of a system using feedback proportional tothe speaker back induced voltage. In FIG. 1, resistor 9 is some fractionof the resistance of the speaker coil 12. Resistor 9 is placed so thatits voltage is out of phase with the speaker voltage. Amplifier 5 hascontrolled gain A=R1/R, where R1 is the resistance of the speaker and Ris the external resistance 9. The output of amplifier 5 is thus avoltage equal to the I R drop in coil 12, and is out of phase with thespeaker voltage. The output of voltage summer 3 in FIG. 1 is only theback induced voltage, since the I R drop portion of the speaker voltagehas been cancelled. This arrangement requires a transformer 8 disposedbetween output amplifier 7 and speaker coil 12, or a transformer as apart of amplifier 7.

FIG. 2 shows a block diagram of a system that does not require atransformer output. In this arrangement the cancelling I R voltage isadded directly to the speaker voltage by superimposing the voltage ofamplifier 19 on the voltage supply to the speaker. The amplifier 19 alsofurnishes the required phase inversion. The output of amplifier 19 isonly the back induced voltage.

The voice coil usually has a small amount of inductance resulting fromwire not in the magnetic field. If desired, the effect of thisinductance can be cancelled by adding inductance to the resistor 9(FIG. 1) or 18 (FIG. 2) in proportion to the ratio of R (9 or 18)divided by the speaker resistance times the inductance of the voicecoil. If R (9 or 18) is one-tenth of the voice coil resistance, theinductance added to R would be one-tenth of the voice coil inductance.

The chief advantage of the circuits described above is that R may bemade small in comparison to R1, thereby giving little power loss, whilederiving the back induced voltage without proportionate voltage loss.The circuit of FIG. 2 has the additional advantage of not requiring atransformer output. Other advantages of these circuits will be discussedunder stability considerations.

To make a practical system work, careful consideration must be given tophase shifts in various stages of the amplifier-speaker system.Practical circuits have phase errors at both high and low frequenciesthat cause the system to oscillate when large amounts of feedback areapplied, unless proper phase-gain relationships are maintained. In orderto avoid oscillation, the gain of the amplifier must be reduced beforeconsecutive phase shifts result in positive feedback.

The stability of the system will be a function of the gain versus phasecharacteristic of the open loop gain of the system. The open loop gainmust be below 1 at frequencies where the phase shift approaches Allpractical amplifier circuits have phase shifts at high and lowfrequencies. It is therefore necessary to decrease the open loop gain ofthe amplifier-transducer system at high and low frequencies.

The back EMF from the speaker is proportional to the velocity of thecone and it, therefore, rises 6 db per octave, as frequency is increasedfor constant cone displacement. Constant cone displacement will giveconstant sound output, at the higher frequencies where the radiationefficiency of the speaker is good, and it is, therefore, desirable tocut the back induced voltage by 6 db per octave of frequency increase,so the resultant voltage will be porportional to the displacement of thecone. This voltage is then used as negative feedback, giving conedisplacement porportional to input voltage.

If a speaker is connected to a constant voltage source, the back inducedvoltage rises 6 db per octave as frequency is increased, until itbecomes a sizeable portion of the total speaker voltage. Increasing thefrequency further causes the back induced voltage to depart from the 6db rising slope and approach the voltage of the driving source. If theback induced voltage is then cut 6 db per octave of rising frequency, asis done to obtain a voltage porportional to cone displacement, theresultant voltage falls 6 db per octave above the frequency where theback induced voltage departs from the 6 db per octave rising slope.Separation of the back EMF, and a 6 db per octave cut of the back EMFwith increasing frequency gives a voltage porportional to thedisplacement of the cone that falls 6 db per octave relative to thespeaker drive with essentially one 90 phase shift at the highfrequencies. This gives an open loop gain, from speaker input tofeedback voltage, that falls 6 db per octave with one 90 phase shift.This characteristic is desirable for the design for high frequencystability.

A constant voltage source can be made by placing inverse feedback on theamplifier that supplies the speaker drive.

Maximum sound produced by constant cone displacement occurs in themid-range and high frequency for most speakers. Below some turn-overfrequency the sound produced by constant cone displacement falls at arate of 12 db per octave. A 6 db boost of the displacement response ofthe system and a 6 db per octave boost in the drive signal will give aconstant sound output below the turn-over frequency. A 6 db per octaveboost can be gained by the natural characteristic of the back EMF.Direct application of the back induced voltage will cause speakerdisplacement to boost 6 db per octave as frequency is decreased. Byletting the R-C network used for high frequency compensation becomeineffective at lower frequencies, the cone displacement response willrise 6 db per octave at lower frequencies, because the back EMF is beingapplied directly. Another 6 db per octave boost may be gained, ifdesired, by another R-C network that reduces the back induced voltage by6 db per octave before it is used for feedback. This would not be used,however, if large amounts of bass feedback are desired. Instead, a 6 dbper octave boost of drive signal would be used. This frequencycompensation is shown at 4 in FIG. 1 and at 20 in FIG. 2. The boost ofthe drive signal is shown at 2 in FIG. 1 and 15 in FIG. 2.

Low frequency phase shifts in conventional amplifier circuits are also aproblem. One low frequency phase shift can be cancelled by allowing aphase lag in the response of the amplifier used to cancel the I Rvoltage portion of the speaker voltage (i.e. amplifier 5 in FIG. 1 andamplifier 19 in FIG. 2). Because of the phase inversion in thisamplifier and the low level of the back induced voltage at lowfrequencies, the induced phase lag causes a phase lead in the voltage atthe output of amplifier 19 in FIG. 2 and in the output voltage of thesummer 3 in FIG. 1, relative to speaker voltage. A phase lag inamplifier 7 or transformer 8 of FIG. 1 can be cancelled, therebyincreasing stability or allowing more feedback.

In FIG. 3 the driver is shown at 50 and the speaker or transducer at 52.The driver has one side connected to one side of the speaker coil 54 byreturn wire 55 and the other side connected to one of the input sides ofa voltage summer 56, the output of which is connected to the input of anamplifier 58. The output of the amplifier is supplied to another voltagesummer 60, the output of which is supplied to one of the input leads ofa frequency compensation component 62. The output of the component 62 isdelivered to the input side of amplifier 64. The output side ofamplifier 64 supplies primary 66 of a transformer 68, which has asecondary coil 70. One side of coil 70 is connected with the side ofspeaker coil 54 opposite its connection to driver 50 while the otherside of coil 70 is connected through a resistor R2 with driver 50 viareturn wire 55 and also by a wire 72 with the other input of voltagesummer 56.

A wire 74 connects the said one side of secondary 70 with the secondinput of voltage summer 60.

The circuit of FIG. 3 includes a return wire 55 to which all componentsare referenced.

Speaker coil 54 has an ohmic resistance equal to R1 and the degree ofamplification of amplifier 58 is equal to R1/R2 times the lossescalculated for voltage summer 56.

The circuit of FIG. 3 represents another way in which the back EMFdeveloped across coil 54 can be fed back into the circuit and applied asnegative feedback to coil 54.

The circuit of FIG. 3, similarly to the circuits of FIGS. 1 and 2,includes a frequency compensation component 62 in the circuit betweendriver 50 and summer 56.

It will be understood that this invention is susceptible to modificationin order to adapt it to different usages and conditions; andaccordingly, it is desired to comprehend such modifications within thisinvention as may fall within the scope of the appended claims.

What is claimed is:

1. In a transducer circuit in which drive amplifier means is driven by asource of audio frequency drive voltage and supplies current to atransducer coil immersed in a magnetic field; that method of controllingthe motion of the coil in the field which comprises: connecting a singleresistor in series with said coil which has a resistance which is afraction of the ohmic resistance of said coil thereby to develop a firstvoltage across said resistor which is a fixed predetermined fraction ofthe IR drop across said coil, amplifying said first voltage at anamplification factor which is the inverse of said fraction and reversingthe phase thereof to produce a second voltage which is substantiallyequal to and in phase opposition with the IR drop across said coil,summing said second voltage with the total voltage drop across said coilto produce a voltage signal which is substantially equal to the back EMFgenerated in the coil by the motion thereof in said field, and supplyingsaid voltage signal to the input side of said amplifier means asnegative feedback.

2. In a transducer circuit: a transducer coil having ohmic resistance R1and immersed in a magnetic field, a first amplifier having input meansand output means, a source of audio frequency drive voltage connectedfor supplying voltage to said input means of said first amplifier, theoutput means of said first amplifier being connected to said coil forsupplying current thereto to cause movement of the coil in the field, asingle resistor connected in series with said coil and having ohmicresistance R2 substantially smaller than R1, means for detecting thetotal voltage drop across said coil and which voltage drop issubstantially equal to the vector sum of the IR drop across the coil andthe back EMF induced in said coil due to movement thereof in said field,means for detecting the voltage drop across said resistor, secondamplifier means having a gain substantially equal to R1/R2 and havinginput means connected to receive the detected voltage drop across saidresistor and having output means at which an amplified voltage isdeveloped which is substantially equal to said IR drop across said coiland substantially in phase opposition therewith, first summing means forsumming said detected total voltage drop across said coil and saidamplified voltage to form voltage signals substantially equal at anyinstant to the back EMF induced in said coil due to the motion thereofin said field, and means for supplying said voltage signals to the inputmeans of said first amplifier for negative feedback of said voltagesignals to said first amplifier, said last mentioned means comprisingsecond summing means receiving said drive voltage and said voltagesignals as inputs and having an output connected to the input means ofsaid first amplifier.

3. A transducer circuit according to claim 2 which includes frequencycompensator means through which the output of said second amplifierpasses prior to the supply thereof to said first amplifier.

4. A transducer circuit according to claim 3 which includes furtherfrequency compensator means through which the supply from said source ofdrive voltage passes prior to the supply thereof to said firstamplifier.

5. In a transducer circuit: a transducer coil having ohmic resistance R1and immersed in a magnetic field, a first amplifier having input meansand output means, a source of audio frequency drive voltage connectedfor supplying voltage to said input means of said first amplifier, theoutput means of said first amplifier being connected to said coil forsupplyingcurrent thereto to cause movement of the coil in the field, asingle resistor connected in series with said coil and having ohmicresistance R2 substantially smaller than R1, means for detecting thetotal voltage drop across said coil and which voltage drop issubstantially equal to the vector sum of the 1R drop across the coil andthe back EMF induced in said coil due to movement thereof in said field,means for detecting the voltage drop across said resistor, secondamplifier means having a gain substantially equal to R1/R2, a firstvoltage summer having a first input terminal connected to said source ofdrive volage and a second input terminal connected to receive thedetected voltage drop across said resistor, said first voltage summerhaving output terminal means connected to the input means of said secondamplifier, and a second voltage summer having a first input terminalconnected to the output means of said second amplifier and a secondinput terminal connected to receive the detected voltage drop acrosssaid coil, said second voltage summer having output terminal meansconnected to the input means of said first amplifier.

6. A transducer circuit according to claim 5 which in- References CitedUNITED STATES PATENTS 2,245,598 6/1941 Llewellyn 179-171 2,843,6715/1958 Wilkins et a1 179-1 2,887,532 5/1959 Werner 179-1 3,096,4887/1963 Lomask 330-107 X 3,187,265 6/1965 Hermes 330-2 KATHLEEN H.CLAFFY, Primary Examiner C. JIRAUCH, Assistant Examiner US. Cl. X.R.330-

